The present invention relates to fluid systems and more particularly an apparatus and method for sensing fluid levels in fluid systems.
The majority of automobiles today employ a float arm and thick-film sender (resistance) card to measure fluid levels, and more specifically fuel levels, in a tank. The float arm follows the fluid level in the tank, which changes the resistance across a sender card. Although this method has proven satisfactory for a number of years, it has a number of movable components that are not robust for assembly. Further, the bulkiness of the float arm and thick-film sender card assemblies limits the accuracy of fluid level measurements in certain fluid tank designs because the bottoms of the assemblies do not rest near the bottom of the tank.
Recently, fluid level sensors have focused on fluid level sensors without any moving parts and which measure hydrostatic pressure to determine fluid height. In a xe2x80x9cno moving parts sensorxe2x80x9d, a pressure sensor is connected to a tube and is open to fluid at the bottom. The air column pressure in the tube will be equal to the hydrostatic pressure caused by the fluid. The height of the fluid can be determined by the pressure reading if the density of the fluid is known. The height that is being measured is the distance between the fluid surface and the meniscus level inside the sensing tube. The meniscus level may change, however, as a function of leakage in the tube/sensor connection, temperature fluctuations, compressibility of the air column in the sensing tube, and/or the tilt angle of the tube. As the meniscus level changes, the distance between the fluid surface and the meniscus level could change.
Also, with current fluid level sensors, the bottom of the fluid tank may expand or contract as a function of fluid weight and tank pressure. As such, a fluid level sensor that is fixed in location in the fluid tank may read more or less depending upon whether the fluid tank is expanding or contracting.
Further, readings on current fluid level sensors may vary due to fluid slosh. As fluid sloshes inside a tank, the height of fluid in various portions of the tank varies. Current fluid level sensors typically average a series of readings to dampen the effect of fluid slosh.
It is thus highly desirable to keep the sensing tube free from fluid or at least lessen the effect of changing meniscus levels in the tube to increase the accuracy of fluid level readings. It is also highly desirable to have the sensing tube measuring fluid levels from the bottom of the fluid tank to increase the accuracy of the measurement of fluid levels. It is also highly desirable to lessen the effect of fluid slosh on fluid level sensor measurements.
It is an object of the present invention to reduce the effect of temperature fluctuations that vary the meniscus levels in the sensing tube. This object may be accomplished in two ways. First, the shape of the tube may be altered to concentrate the volume of the tube near the bottom. In this way, large temperature fluctuations will cause only slight variation in the meniscus level of the tube. Second, a diaphragm may be added to the end of the tube that transmits hydrostatic fluid pressure into the air column, but does not allow fluid to enter the tube. In this way, the depth of the fluid in the fluid tank is always measured from the bottom of the tube to the fluid surface, greatly increasing the accuracy of the measurement.
It is another object of the present invention to provide a xe2x80x9cbottom-sensingxe2x80x9d design in the tube that will force the tube to rest approximately on the bottom of the fluid tank. In this way, the measurement of fluid level will be closer to the true height of the fluid even if the fluid tank deforms due to the weight of the fluid or tank pressure/vacuum.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.